Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/133

 afterwards taken up by David Brewster (b. 1781, d. 1868), who in 1815 showed that there is complete polarization by reflexion when the reflected and refracted rays satisfy the condition of being at right angles to each other.

Almost at the same time Brewster made another discovery which profoundly affected the theory of double refraction. It had till then beer believed that double refraction is always of the type occurring in Iceland spar, to which Huygens' construction is applicable. Brewster now found this belief to be erroneous, and showed that in a large class of crystals there are two axes, instead of one, along which there is no double refraction. Such crystals are called biaxal, the simpler type to which Iceland spar belongs being called uniaxal.

The wave-theory at this time was still encumbered with difficulties. Diffraction was not satisfactorily explained; for polarization no explanation of any kind was forthcoming; the Huygenian construction appeared to require two different luminiferous media within doubly refracting bodies; and the universality of that construction had been impugned by Brewster's discovery of biaxal crystals.

The upholders of the emission theory, emboldened by the success of Laplace's theory of double refraction, thought the time ripe for their final triumph; and as a step to this, in March, 1817, they proposed Diffraction as the subject of the Academy's prize for 1818. Their expectation was disappointed; and the successful memoir afforded the first of a series of reverses by which, in the short space of seven years, the corpuscular theory was completely overthrown.

The author was Augustin Fresnel (b. 1788, d. 1827), the son of an architect, and himself a civil engineer in the Government service in Normandy. During the brief dominance of Napoleon after his escape from Elba in 1815, Fresnel fell into trouble for having enlisted in the small army which attempted to bar the exile's return; and it was during a period of enforced idleness following on his arrest that he commenced to study